O-2-promoted allylic acetoxylation of alkenes: Assessment of 'push' versus 'pull' mechanisms and comparison between O-2 and benzoquinone

摘要

Palladium-catalyzed acetoxylation of allylic C-H bonds has been the subject of extensive study. These reactions proceed via allyl-palladium(II) intermediates that react with acetate to afford the allyl acetate product. Benzoquinone and molecular oxygen are two common oxidants for these reactions. Benzoquinone has been shown to promote allyl acetate formation from well-defined pi-allyl palladium(II) complexes. Here, we assess the ability of O-2 to promote similar reactions with a series of "unligated" pi-allyl palladium(II) complexes (i.e., in the absence of ancillary phosphorus, nitrogen or related donor ligands). Stoichiometric and catalytic ally' acetate formation is observed under aerobic conditions with several different alkenes. Mechanistic studies are most consistent with a "pull" mechanism in which O-2 traps the Pd-0 intermediate following reversible C-O bond formation from an allyl-palladium(II) species. A "push" mechanism, involving oxidatively induced C-O bond formation, does not appear to participate. These results and conclusions are compared with benzoquinone-promoted allylic acetoxylation, in which a "push" mechanism seems to be operative. (C) 2014 Elsevier Ltd. All rights reserved.